User operated drug delivery devices are as such known in the prior art. They are typically applicable in circumstances, in which persons without formal medical training, i.e., patients, need to administer an accurate and predefined dose of a medicinal product, such as heparin or insulin. In particular, such devices have application, where a medicinal product is administered on a regular or irregular basis over a short term or long-term period.
In order to accommodate with these demands, such devices have to fulfill a number of requirements. First of all, the device must be robust in construction, yet easy to use in terms of handling and in understanding by the user of its operation and the delivery of the required dose or medicament. The dose setting must be easy and unambiguous. Where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose. Moreover, the device should be suitable for recycling. To meet these requirements, the number of parts required to assemble the device and the number of material types the device is made from need to be kept to a minimum.
The medicinal product to be dispensed by means of the drug delivery device is typically provided in a disposable or replaceable cartridge, such as a vial, an ampoule or a carpule comprising a slidably disposed piston to be operably engaged with a piston rod of the drug delivery device's drive mechanism. By applying thrust to the cartridge's piston in distal direction, a predefined dose of the liquid drug can be dispensed and expelled from the cartridge.
Cartridges as they are typically used with drug delivery devices are typically sealed by means of as sealing septum. Such a septum is commonly designed as rubber stopper providing an air-tight seal but being pierceable by piercing elements such as needles or cannulae.
A typical cartridge holder assembly 10 according to the prior art is illustrated in cross section in FIG. 1. The cartridge holder assembly 10 comprises a cartridge holder 14, 16 adapted to receive a cartridge 12, which is hermetically sealed with a flexible and deformable septum 18. At its lower and distal end section 14, the cartridge holder is threadedly engaged with a needle mount 24. Said mount or needle holder comprises a threaded cylindrical portion 24 allowing to screw the needle holder on the threaded neck portion 14 of the cartridge holder. At its lower and distal section, the mount 24 comprises a bottom portion 25, which in a concentrically inner section holds the injection needle or cannula 30.
During assembly of the needle mount 24, the proximally located tipped end of the needle 30 penetrates the septum 18. In this way, a fluid-transferring connection for the purpose of dose dispensing can be established. Additionally, the distal and free end of the needle 30 can be provided with a removable needle cap 28. Also, the entire cartridge holder assembly 10 can be covered and protected by a protective cap 26.
Depending on manufacturing tolerances and depending on the concrete design of cartridge 12 and cartridge holder 14, 16, an axial gap 22 of variable size is typically formed between the bottom portion 25 of the needle mount 24 and the distal end face of the cartridge 12. As further illustrated in FIG. 1, the needle mount 24 comprises a radially inwardly directed bottom portion 25 at its distal end, which serves as a distal end stop for the cartridge 12. With an aluminium cap 20 that fixes the septum 18 in position, the cartridge 12 buts against said bottom portion 25.
Axial size of said space 22 may vary, e.g. due to manufacturing and assembly tolerances. In particular, during dispensing of a dose of a medicinal fluid, a respective fluid pressure is built-up, which, due to the elasticity of the septum 18, may lead to a respective axial expansion of the septum 18. As a consequence, the septum 18 may at least partly extend through a through opening 23 of the aluminium cap 20. When a respective fluid pressure builts up inside the cartridge 12 the septum 18 may almost entirely fill out the space 22 between the needle holder 25 and the distal end face of the cartridge 12.
Due to its elastic properties, the septum 18 also stores elastic energy during dose dispensing. As soon as the fluid pressure returns to an initial level after termination of a dose dispensing procedure, the septum 18 relaxes to its initial configuration, which is accompanied by a retraction of the expanded section of the septum 18 back into the cartridge 12. Such retracting motion may in turn lead to a built-up of a non-negligible post-dispensing fluid pressure, and, as a consequence, a certain amount of medicinal fluid may be supplementally expelled from the cartridge 12, which can be typically observed in the form of droplet formation at the distal tip of the needle 30.
Moreover, the axial expansion of the septum 18 may also lead to a respective proximally directed axial displacement of the cartridge 12 with respect to the cartridge holder 14 and/or with respect to the housing of the drug delivery device. Additionally, the drive mechanism operably engaged with the cartridge and/or its piston may also become subject to a respective axial displacement due to manufacturing and/or assembly tolerances. Generally, any axial displacement of the cartridge and/or its septum is finally at the expense of dosing accuracy.